This invention relates to a data communication channel and in particular to locating and identifying conditions, including nonlinear impairments, affecting the transmission characteristics of the channel. This invention could potentially also relate to any linear medium that supports wave propagation and can contain sites where nonlinear impairments affect transmission.
The rapid expansion of the Internet has resulted in an increased demand for a global high-speed data network for communication between Internet subscribers and ISPs. In response to the demand, the worldwide telephone network and the cable television network have been adapted to provide this service. While much of the telephone network has been adapted to support advanced digital communications between central offices, the link from a central office to the premises of a local subscriber is still predominantly the twisted pair copper loop (also called a “subscriber loop”). At present, there are in place many miles of twisted pair copper subscriber loops that were originally designed to carry low frequency analog voltage voice signals. To meet the Internet requirement of transferring packetized digital information, new encoding and modulation methods have evolved to carry higher digital data rate transmissions over these low frequency telephone network loops. For example, Digital Subscriber Line systems (DSL), share in common accepting digital inputs from a subscriber's or an ISP's computer, converting it to a complex analog signal for high speed transmission over the telephone network, and reconverting it to Internet compatible digital information at the receiving site. Successful high-speed transmission of digital information relies on maximum utilization of the capabilities of the transmission channel, and this requires matching the DSL system requirements to the existing telephone channel characteristics.
Cable television channels have also being adapted for the transmission of data, and may exhibit problems similar to those experienced in data transmission over the telephone network. These transmissions primarily occur on coaxial cables consisting of an inner conducting wire surrounded by a conducting shield.
The channels used for transmission of DSL signals are conventionally configured, with channel output being a linear function of channel input, so that even if signals are subject to linear distortion they can be recovered using straightforward equalization techniques. However, a channel that is otherwise entirely linear may be subject to nonlinear effects that occur at one or more discrete locations in the channel. For example, a line driver at the channel input may exhibit some nonlinear distortion, an imperfect contact or splice in the channel may have nonlinear characteristics, or a transformer in the channel may exhibit a significant nonlinearity related to magnetic hysteresis and saturation in the core. These nonlinearities give rise to intermodulation components that become dispersed in time as the signals traverse the channel due to the phase shift of the linear portion of the channel. At the receiving end of the channel these components arrive intermixed relative to each other, and cannot be compensated for or corrected by the linear equalizers known in the art.
Channel equalization for compensation of signal distortion produced both linear and nonlinear impairments is accordingly a priority requirement for efficient, effective DSL operation over a telephone or cable network. U.S. application Ser. No. 09/968,063 filed Oct. 1, 2001 in the name of Bryant, discloses a method and apparatus for correction of both linear and nonlinear distortion. U.S. Application 60/422,655 filed Oct. 30, 2002 in the name of Bryant discloses a method and apparatus for echo cancellation in lines having nonlinear impairments. Both of these applications are hereby incorporated by reference.
It is known in the art that time domain reflectometry (TDR) can detect the presence and indicate location of irregularities in a channel by measurement of the return time of pulses reflected from line impairments. However, the method does not distinguish between linear and nonlinear irregularities. U.S. Pat. No. 6,275,050 discloses detection of nonlinear effects caused by corrosion in metal junctions by the measurement of harmonics and intermodulation products generated in signals which have passed through corroded metal junctions.
Referring to FIG. 1, a TDR readout trace 10, as practiced in prior art, shows a main pulse 12 and noise pulses, e.g. 14, derived from the voltages present on a communication channel having no impairments. The main pulse 12 is connected or coupled to a transmission channel being tested. The channel may include one or more linear or nonlinear impairments which cause reflections of the main pulse 12. These reflections (also “echoes”) travel through the channel against the direction in which the pulse 12 travels, arriving back at the point or location at which the pulse was introduced into the channel. For example, in FIG. 2, the main pulse 12′ results in a reflection or echo 16 from a linear impairment, and a reflection or echo 18 from a nonlinear impairment, as well as noise signals 14′. (In the drawings, equivalent elements are identified with the same reference numbers, albeit they are distinguished by primes.) It will be noted that the echo 16 from the linear impairment, and the echo 18 from the nonlinear impairment both appear on the trace 10′, and are not distinguishable. The traces 10 and 10′ display the main pulse, the noise and reflected signals, in a manner known in the art. It will be noted that, in practice, the echo pulses, e.g. 16, 18 will often be much smaller in height that the main pulse 12′.
This invention may be embodied in a method or an apparatus for detecting the presence and locations of linear and nonlinear impairments in a transmission channel, and for differentiating the nonlinear impairments from the linear ones. Preferably, the effects produced in echoes from one or more nonlinear impairments are distinguished from and cancel the effects produced in echoes from one or more linear impairments so as to reveal the presence and locations of any nonlinear impairment. I call this invention nonlinear time domain reflectometry (NTDR).